linux-kernel - [PATCH] dynamically allocate struct cpumask

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Message-Id: <20200127071602.11460-1-nick.desaulniers@gmail.com>
Date:   Sun, 26 Jan 2020 23:16:02 -0800
From:   Nick Desaulniers <nick.desaulniers@...il.com>
To:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de
Cc:     Nick Desaulniers <nick.desaulniers@...il.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: [PATCH] dynamically allocate struct cpumask

This helps avoid avoid a potentially large stack allocation.

When building with:
$ make CC=clang arch/x86/ CFLAGS=-Wframe-larger-than=1000
The following warning is observed:
arch/x86/kernel/kvm.c:494:13: warning: stack frame size of 1064 bytes in
function 'kvm_send_ipi_mask_allbutself' [-Wframe-larger-than=]
static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int
vector)
            ^
Debugging with:
https://github.com/ClangBuiltLinux/frame-larger-than
via:
$ python3 frame_larger_than.py arch/x86/kernel/kvm.o \
  kvm_send_ipi_mask_allbutself
points to the stack allocated `struct cpumask newmask` in
`kvm_send_ipi_mask_allbutself`. The size of a `struct cpumask` is
potentially large, as it's CONFIG_NR_CPUS divided by BITS_PER_LONG for
the target architecture. CONFIG_NR_CPUS for X86_64 can be as high as
8192, making a single instance of a `struct cpumask` 1024 B.

Signed-off-by: Nick Desaulniers <nick.desaulniers@...il.com>
---
 arch/x86/kernel/kvm.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 32ef1ee733b7..d41c0a0d62a2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -494,13 +494,15 @@ static void kvm_send_ipi_mask(const struct cpumask *mask, int vector)
 static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
 {
 	unsigned int this_cpu = smp_processor_id();
-	struct cpumask new_mask;
+	struct cpumask *new_mask;
 	const struct cpumask *local_mask;
 
-	cpumask_copy(&new_mask, mask);
-	cpumask_clear_cpu(this_cpu, &new_mask);
-	local_mask = &new_mask;
+	new_mask = kmalloc(sizeof(*new_mask), GFP_KERNEL);
+	cpumask_copy(new_mask, mask);
+	cpumask_clear_cpu(this_cpu, new_mask);
+	local_mask = new_mask;
 	__send_ipi_mask(local_mask, vector);
+	kfree(new_mask);
 }
 
 /*
-- 
2.17.1